The present invention relates to a method of measuring the static magnetic field distribution in a nuclear magnetic resonance (NMR) inspection system which measures the nuclear spin distribution and relaxation time distribution inside a 3-dimensional substance such as a biological substance by utilization of the NMR phenomena.
The NMR phenomena discovered in 1946 coincidently by Bloch and Purcell are now indispensable means of analysis in the fields of physics and chemistry, including the structural analysis of substances.
Application of the NMR phenomena to the imaging technique has first been attempted by Lauterbur in 1974. Thereafter, various NMR imaging methods have been developed, and at present the pulse method providing a satisfactory measuring accuracy and S/N ratio is dominant.
In the NMR imaging technique, it is necessary to have a uniform static magnetic field distribution so as to obtain images without distortion. For measuring the static magnetic field distribution, there is known the 3-dimensional Fourier transform imaging method. (Refer to A. A. Maudsley et al, "Rapid Measurement of Magnetic Field Distributions Using Nuclear Magnetic Resonance", Siemens R/D Report vol. 8, pp. 326-331 (1979)).
FIG. 1 shows the sequence for practicing the above-mentioned method. In the figure, G.sub.x and G.sub.y represent the phase-encoding gradients, and the image is formed on the x-y plane. The 3-dimensional Fourier transformation for 3-dimensional signal F(G.sub.x, G.sub.y, t) obtained through the measurement gives NMR spectrum at each point of the image. Accordingly, by plotting the frequencies at peaks of the spectrum at each point of the image using a phantom containing water uniformly, the static magnetic field can be mapped.
Although the above method is advantageous in that the disparity of the sensitivity of the irradiation and detection coils does not affect the measurement result, it needs the observation of at least as many FID signals as the number of measuring points, resulting in a problem of a longer measurement time.